The present invention relates generally to diagnostic imaging and, more particularly, to a method and apparatus for high flux rate imaging with energy discrimination, such as in computed tomography (CT) applications.
Diagnostics devices comprise x-ray systems, magnetic resonance (MR) systems, ultrasound systems, computed tomography (CT) systems, positron emission tomography (PET) systems, and other types of imaging systems. Typically, in CT imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry opening within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. The photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction. A characteristic of using scintillators and photodiodes is that the output signal passed to the system is proportional to the x-ray energy incident to the detector during an integration view time. Some systems provide, instead, a photon counting detection mechanism whereby the signal passed to the system is proportional to the number of x-ray photons incident to the detector during the integration view time. Since the spectrum of the incident x-ray flux has a breadth in energy, there is a considerable difference in the number of x-ray photons and the total energy of the photons. Thus, a photon counting detector has improved dose efficiency. In addition, the photon counting detector can be configured with additional energy thresholds to distinguish photons at different energy thresholds and count these in separate registers. Generally, photon counting systems used for CT imaging use direct conversion sensor materials because the signal charge created per x-ray is much greater than that of a scintillator/photodiode sensor.
A CT imaging system comprises an energy discriminating (ED) and/or multi energy (ME) CT imaging system that may be referred to as an EDCT and/or MECT imaging system. The EDCT and/or MECT imaging system may be configured to be responsive to different x-ray spectra. For example, a conventional third generation CT system acquires projections sequentially at different x-ray tube potentials. Two scans are acquired either back to back or interleaved in which the tube operates at 80 kVp and 160 kVp potentials. Special filters are placed between the x-ray source and the detector such that different detector rows collect projections of different x-ray energy spectra. The special filters that shape the x-ray spectrum in an example can be used for two scans that are acquired either back to back or interleaved. Energy sensitive detectors are used such that each x-ray photon reaching the detector is recorded with its photon energy.
Exemplary ways to obtain energy sensitive measurements comprise: (1) scan with two distinctive energy spectra, (2) detect photon energy according to the depth from the incident surface for energy deposition in the detector, and (3) photon counting with multiple energy thresholds. EDCT/MECT provides energy discrimination and material characterization. For example, in the absence of object scatter, the system derives the behavior at any other energy based on the signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In an energy region of medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two materials.
A conventional basis material decomposition (BMD) algorithm is based on the concept that in the energy region for medical CT, the x-ray attenuation of any given material can be represented by a proper density mix of two other materials, referred to as the basis materials. Based on the projections acquired at the two incident x-ray spectra, the BMD algorithm computes two sets of new projections, corresponding to two new CT images that each represents the equivalent density of one of the basis materials. Since a material density is independent of x-ray photon energy, these images are approximately free of beam-hardening artifacts. An operator can choose the basis material to target a certain material of interest, for example, to enhance the image contrast.
A previous photon counting detector saturates at high flux rate and degrades due to pile-up loss of detector quantum efficiency (DQE) and spectral information. Direct conversion semiconductor detectors that comprise high atomic number elements like cadmium telluride (CdTe) or cadmium zinc telluride (CZT) may suffer from polarization, for example, when operating in either the integration or photon counting mode. Other materials like silicon (Si) or gallium arsenide (GaAs) may have a crystal structure with fewer defects. These crystals may not polarize at high flux and have high mobility and therefore a high flux rate counting capability. Semiconductor layers such as silicon and GaAs may be employed as direct conversion radiation detectors operated in either the integration or photon counting mode. A thin layer of these low atomic number materials may not stop a substantial fraction of the flux. These materials as crystals provide low attenuation and stop a relatively small fraction of the photons in the beam of x-rays 16.
An energy discriminating (ED) detector may comprise a plurality of integrating layers. It is desirable for dose efficiency to have the total x-ray spectra that is incident to the detector to be absorbed in the combination of the layers. Conventionally, two scintillator/photodiode layers are used where the low energy portion of the spectra is absorbed in a thin top layer and high energy portion of the spectra is absorbed in a thick bottom layer. Top and bottom layers may, instead, comprise direct conversion layers, for example, with a relative thickness in a layer that may lead to polarization effects, instability, and/or non-linear responses at relatively high x-ray flux. Thick layers of direct conversion material require a longer transport distance for charge carriers and therefore a greater amount of charge trapping. An ED detector may comprise a plurality of scintillator and photodiode layers, for example, with interaction between the top and bottom layers that may add noise for the low and high energy signal.
Layered detectors may be challenging, difficult, and/or problematic, for example, because the ASIC parts, bonding pads, interconnect, and substrate materials between the top and bottom layers may attenuate the flux to the bottom layer without creating and/or contributing a signal that is employable and/or appropriate in image reconstruction. Coupling of the readout ASIC directly to the back of the top layer sensor may risk radiation damage to the ASIC and attenuation of the beam of x-rays 16 in traversal though the top layer to the bottom layer.
Alternately, an ED detector may comprise a single photon counting layer. At low incident flux, a single-layer, photon counting detector will generally give material basis decomposition at lower dose than the ED detector composed of multiple integrating layers. Such a layer needs to be thick enough such that it absorbs a substantial fraction of the incident x-ray flux. The relatively thick layer may lead to polarization effects, instability, and/or non-linear response at relatively high x-ray flux. Furthermore, a relatively thick, single layer photon counting detector will saturate due to pile-up when x-ray photons arrive at the detector faster than the readout electronics can register them.
Therefore, it would be desirable to design an apparatus and method to promote a reduction in one or more of energy discriminating (ED) detector layer thickness, ED detector polarization, instability, non-linearity, and/or noise. Further, it would be desirable to design an apparatus that has the improved dose efficiency of a photon counting system but that also has a means to mitigate the saturation phenomenon of pile-up.